Anonymous ,"Cavendish: On best form for two years ahead of Tour de France" This seems to be a headline and not a sentence. "He's on his best form" is less common that "He's in his best form," but it's acceptable, in my opinion. Exactly what your quote means with respect to time factors is open to interpretation, unless you happen to be familiar with this particular sportswriter's style.
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Anonymous,"Cavendish: On best form for two years ahead of Tour de France"This seems to be a headline and not a sentence.